Recently, there has been growing interest in energy storage technologies. As energy storage technologies are extended to devices such as cellular phones, camcorders and notebook PC, and further to electric vehicles, demand for high energy density of batteries used as a source of power supply of such devices is increasing. Therefore, research and development of lithium secondary batteries, which most meet the demand, are actively being conducted.
Among secondary batteries currently used, a lithium secondary battery developed in the early 1990's comprises an anode made of carbon material capable of intercalating or disintercalating lithium ions, a cathode made of lithium-containing oxide, and a non-aqueous electrolyte solution obtained by dissolving a suitable amount of lithium salt in a mixed organic solvent.
Since the use of lithium secondary batteries is extended from conventional small electronics to large-sized electronic equipments, vehicles, smart grid systems and the like, demands for a lithium secondary battery capable of maintaining its good performances under severe external conditions such as low or high temperature as well as room temperature are gradually increasing.
Conventionally, many non-aqueous electrolyte solutions have been prepared by using carbonate-based organic solvents such as ethylene carbonate (EC) and dimethyl carbonate (DMC) as a solvent for the electrolyte solutions.
However, the carbonate-based organic solvents may be decomposed on the surface of an electrode during charging and discharging processes to cause a side reaction in batteries, and solvents having large molecular weight, such as ethylene carbonate (EC), dimethyl carbonate (DMC) and diethyl carbonate (DEC) are intercalated between graphite layers in a carbon-based anode during an initial charging process, from which the structure of the anode may be degraded. Accordingly, as charging and discharging processes are repeated, the performances of lithium secondary batteries may be deteriorated.
Such a problem is known to be solved by a solid electrolyte interface (SEI) layer which is formed on the surface of an anode by the reduction of a carbonate-based organic solvent, but the SEI layer known until now is insufficient as a film for continuously protecting an anode. That is, as a surface reaction on an anode is continuously carried out, the capacity of batteries may decrease and the life characteristic thereof may be deteriorated. Also, during the formation of the SEI layer, the carbonate-based organic solvent may decompose to generate a gas such as CO, CO2, CH4 and C2H6, which may cause a swelling phenomenon resulting in the deterioration of batteries. The decomposition gas thus generated may deform pouch- or can-type battery assembly to cause an internal short circuit, and in severe cases, batteries may ignite or explode.
In order to overcome such problems, various additives such as vinylene carbonate, saturated sultone and unsaturated sultone have been proposed with the purpose of preventing batteries from being subject to swelling.
However, when a certain compound is added to an electrolyte solution so as to improve the performances of batteries, some properties may be improved but other properties may be reversely deteriorated. For example, in the case of vinylene carbonate, it may be decomposed prior to conventional organic solvents, thereby forming an SEI layer, however it has a problem of being easily decomposed on a cathode under the environment of a high temperature, thereby generating a gas.
Accordingly, there is a still need to develop a non-aqueous electrolyte solution capable of minimizing such problems.